1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor substrate having a semiconductor layer on its insulating surface.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) using a semiconductor thin film (the thickness of several nm to several hundreds of nm) formed over a substrate having an insulating surface such as glass has been attracted. Thin film transistors are applied to a wide range of electronic devices like integrated circuits and electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed.
Instead of silicon wafers that are manufactured by thinly slicing an ingot of a single crystal semiconductor, semiconductor substrates called silicon-on-insulator (SOI) substrates have been developed, which have a thin single crystal semiconductor layer over an insulating layer, and are more widely used as substrates in manufacturing microprocessors or the like. This is because an integrated circuit using an SOI substrate draws attention as an integrated circuit in which parasitic capacitance between drains of transistors and a substrate can be reduced, performance of the semiconductor integrated circuit can be improved, and low power consumption is achieved.
As a method for manufacturing SOI substrates, a hydrogen ion implantation separation method is known (for example, Reference 1: Japanese Published Patent Application No. 2000-294754). For example, ions of hydrogen or the like are implanted to a silicon wafer provided with a silicon oxide film on its surface to form a microbubble layer serving as a cleavage plane at a predetermined depth from the surface, and a thin film single crystal silicon layer (SOI layer) is bonded to another silicon wafer. Then, the SOI layer is separated from the cleavage plane by heat treatment to form an SOI substrate. The hydrogen ion implantation separation method is also called a Smart Cut (registered trademark) method.
In Reference 1, after the separation, the SOI layer separated from the silicon wafer is irradiated with laser light to planarize the SOI layer.
Meanwhile, a method for forming an SOI layer over an insulating substrate such as glass has been attempted. As an example of SOI substrates in which an SOI layer is formed on a glass substrate, an SOI substrate in which a thin single crystal silicon layer is formed over a glass substrate having a coating film by a hydrogen ion implantation separation method is known (for example, Reference 2: Japanese Published Patent Application No. 2004-134675). In this case also, a thin single crystal silicon layer (SOI layer) is formed on the glass substrate in such a way that a microbubble layer is formed at a predetermined depth from the surface by implantation of hydrogen ions to a silicon wafer, the glass substrate and the silicon wafer are bonded to each other, and the silicon wafer is separated using the microbubble layer as a cleavage plane.